The present invention is directed to a dental turbine drive which contains a rotor disc guided in bearings. The rotor disk comprises a plurality of turbine buckets on the periphery of the rotor that can be charged with compressed air via a nozzle disposed in the housing of the handpiece and the turbine drive includes adjustment means for controlling the turbine speed being disposed in the flow path of the exhaust air from the turbine.
Turbine drives have been utilized for a long time in dental turbine handpieces and are distinguished over other drives by a relatively simple structure as well as a comparatively simple arrangement for supplying power to the drive. However, they are affected by the disadvantage of a relatively low eroding power and what is usually an excessively high no-load speed, which may extend up to a maximum of 400,000 rpm. When a load is applied to the tool being driven, the speed of the turbine will abruptly drop. The high speed will also cause a relatively high wear of the bearings, which may be either plain bearings or ball bearings. Moreover, there is a risk that the eroded material, for example a dental substance, will heat to an inadmissible extent or will even be burned given the relatively high speed of the turbine as the tool, such as a drill, first contacts the material to be eroded, such as a tooth.
German Patent 845 092 discloses a dental turbine drive and the speed of the turbine can be varied by varying the cross section of outlet openings for the compressed air flowing off from the turbine. The discharge channel is formed between two cylindrical parts, which are arranged concentrically relative to one another, and each of these parts has a plurality of openings on a circumference. Thus, a variation of the cross section of the outlet openings can be obtained by displacing the outer cylindrical part or sleeve on the inner part so as to change the amount of coincidence of the openings of the two parts by either increasing the coincidence or decreasing the coincidence. The displacement of the outer cylindrical part occurs manually with a lever that extends along the outside of the handpiece.
The arrangement of this publication discloses a possibility of setting a speed desired by the user of the handpiece but does not disclose any measures for an automatic regulation of the speed depending on the load being applied to the tool rotated by the turbine drive.
U.S. Pat. No. 3,865,505, whose disclosure is incorporated herein by reference thereto, discloses control means for a dental turbine wherein a valve that is controlled dependent on the volume throughput of the returned air is arranged in the delivery channel for the compressed air being introduced to the turbine drive. When the speed of the turbine drops due to an external load, then the valve in the delivery channel is opened and a greater volume of air is conducted to the rotor disc of the turbine drive. The control of the inlet air stream dependent on the returned air can occur in various ways in the turbine of this patent. For example, among other things, a spring-loaded slide in the return air channel will control a valve in the inlet air channel or a membrane or diaphragm is arranged in the return air channel which will displace a piston that will control the flow in the inlet channel. The speed can be kept constant independently of load with this known control means. The apparatus has, among other things, the disadvantage that relatively great forces are necessary for throttling the overall air stream, and these forces can be produced in the exhaust air channel only given employment of volume and mass. However, an unstable control behavior of the turbine is, thus, created. In addition, the known device requires a relatively large structure within the turbine handpiece and, therefore, integration problems can arise as a result of this necessary size.
U.S. Pat. No. 3,578,872, whose disclosure is incorporated herein by reference thereto, discloses a two-stage turbine drive provided for a drive of a surgical tool. The drive is provided with measures, on the one hand, to limit the maximum speed of the turbine in order to thereby avoid what is referred to as overspeeding of the turbine and, in addition, in order to increase the torque under load. The measures provided for this purpose are composed of an annular part rotating together with the turbine wheel that has a circumferential rim on the periphery and an annular space for receiving an elastic ring is formed between this peripheral rim and part of the rotor or armature sleeve of the part. The elastic ring will expand with increasing speed as a consequence of the centrifugal force and thereby will close openings provided on an end face of the part so that the throughput volume of the air will be reduced. The effective cross section can be varied by these measures and the maximum speed of the turbine can be ultimately limited.
The setting or, respectively, variation of speed here occurs in a known way by varying the volume of the supplied air on the basis of an additional manually-actuatable control valve that is not shown in that patent.